1. Field of the Invention
This disclosure relates to a catalyst for a fuel cell and a membrane-electrode assembly, and a fuel cell system including the same.
2. Description of the Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical oxidation-reduction reaction of an oxidant and hydrogen included in a hydrocarbon-based material such as methanol, ethanol, natural gas, and the like. Such a fuel cell is a clean energy source that can replace fossil fuels. It includes a stack of unit cells, and has an advantage of being able to produce various ranges of power. Since it has a four to ten times higher energy density than a small lithium battery, a fuel cell has been touted as a small portable power source.
Typical examples of a fuel cell are a polymer electrolyte membrane fuel cell (“PEMFC”) and a direct oxidation fuel cell (“DOFC”). A direct oxidation fuel cell that uses methanol as a fuel is called a direct methanol fuel cell (“DMFC”). The polymer electrolyte fuel cell has an advantage of having high energy density and power, but also has the difficulties associated with requiring accessory facilities such as a fuel-reforming processor for reforming a fuel gas such as methane, methanol, and natural gas to produce hydrogen and also the difficulties associated with handling the hydrogen gas. The direct oxidation fuel cell has lower energy density than the polymer electrolyte fuel cell, but has advantages of easy handling of a liquid fuel, operating at a low temperature, and requiring no additional fuel reforming processor.
In one of the above-described fuel cells, the stack generating electricity includes several unit cells stacked in multiple layers. Each unit cell is made up of a membrane-electrode assembly (“MEA”) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly has an anode (referred to as a fuel electrode or an oxidation electrode) and a cathode (referred to as an air electrode or a reduction electrode) attached to each other with an electrolyte membrane therebetween. Fuel is supplied to the anode, is adsorbed on anode catalysts, and then is oxidized to produce protons and electrons. The electrons are transferred into the anode, which is a reducing electrode, via an external circuit, while the protons are transferred into the cathode through the polymer electrolyte membrane. In addition, an oxidant is supplied to the cathode. Then the oxidant, protons, and electrons react on cathode catalysts to produce electricity and water.